The present invention relates to thermoprocessable perfluorinated polymers for the preparation of manufactured articles used in the semicon industry, which show very low values of extractable Fxe2x88x92, both on perfluoropolymer pellet and the manufactured articles therefrom.
Specifically, the present invention relates to thermoprocessable copolymers of tetrafluoroethylene (TFE) which show very low values of extractable Fxe2x88x92 combined with very good mechanical and elastomeric properties combine wherefore they result particularly suitable to be used in the semicon industry in the preparation of pipes, fittings and tanks for the storage and the transportation of chemical compounds and of ultrapure water. Besides, the thermoprocessable copolymers of tetrafluoroethylene (TFE) of the invention show very low values of extractable Clxe2x88x92, lower than 0.2 ppm by weight with respect to the polymer weight, preferably lower than the analytical detection limits.
It is well known that in the semicon industry tanks and piping systems (pipes and fittings) are made of fluorinated polymer materials which during the use must release a minimum amount of Fxe2x88x92, lower than or equal to 1 ppm, so as not to contaminate the transported fluids, thus avoiding the damaging of silicon-based wafers. In the paper xe2x80x9cFluoride Contamination from Fluoropolymers in Semiconductor Manufacturexe2x80x9d published on xe2x80x9cState Solid Technologyxe2x80x9d pages 65-68, July 1990, it is stated that for the semicon industry it would be desirable to obtain finished manufactured articles which during the use release a minimum amount of Fxe2x88x92, in particular lower than or equal to 1 ppm.
The manufactured articles for the semicon industry are generally prepared with thermoprocessable copolymers of TFE and perfluoropropylvinylether (PPVE), belonging to the PFA class, wherein PPVE is about 3.5-4.5% by weight. Said copolymers are preferably obtained by a polymerization process in aqueous emulsion which allows to obtain a high productivity and the formation of polymer structures having a high molecular weight, therefore characterized by good mechanical properties and high ductility. However the TFE/PPVE copolymers contain a certain amount of chain end groups of ionic type xe2x80x94CF2COOH and of xe2x80x94COF type. The xe2x80x94COF end group, as a consequence of hydrolysis reactions which occur during the polymerization itself, can be transformed into ionic xe2x80x94CF2COOH end group. In the processing, for example for the preparation of pipes or fittings, said end groups can decompose producing hydrofluoric acid HF. The formed hydrofluoric acid is released during the time from the manufactured article during the use in the semicon production plant, causing an unacceptable corrosion of silicon-based wafers. To minimize the HF formation during the processing so that the finished manufactured article shows the minimum amount of extractable Fxe2x88x92, the prior art uses a fluorination process to transform the end groups into stable perfluorinated groups. See for example U.S. Pat. No. 4,743,658, wherein the TFE/PPVE copolymer is subjected to fluorination with elemental fluorine for reducing the amount of ionic end groups so that the fluorine extractable from the finished manufactured article is lower than 3 ppm by weight with respect to the polymer. Said process requires an additional step and from the industrial point of view it is not easily feasible since it uses elemental fluorine, very aggressive agent which requires special equipments for making the treatment safe and reliable.
Other processes for reducing the ionic end group number in perfluorinated copolymes are known. See for example U.S. Pat No. 5,093,409 wherein the TFE/PPVE copolymer under the form of latex is treated with amines at 160xc2x0-400xc2x0 C. for a sufficient time to convert the ionic xe2x80x94CF2COOH end groups into xe2x80x94CF2H groups. Also this post-treatment for stabilizing the fluoropolymer requires a specific unit and it results therefore expensive from the economic point of view.
A class of thermoprocessable copolymers of TFE having very high chemical inertia and thermal stability is described in U.S. Pat. No. 5,463,006, wherein terpolymers formed by TFE/PPVE/PMVE (perfluoromethylvinylether) are described. Tests carried out by the Applicant, see the comparative Examples, show that with these terpolymers release values of Fxe2x88x92 lower than 1 ppm are not obtained both on the polymer (pellet) and on the finished manufactured article. Therefore also in this case it is necessary to carry out one of the above post-treatments for reducing the values of extractable Fxe2x88x92.
In conclusion, the thermoprocessable TFE copolymers of the prior art used for pipes in the semicon industry show a very good combination of properties, such high flex life, good mechanical properties at high and low temperature and release values of Fxe2x88x92 lower than 1 ppm. However the drawback of said products is that the combination of said properties is obtained with the proviso to subject them to a specific treatment of fluorination or conversion of the end groups of the previously described type. All this requires a further processing step and an additional specific unit, making therefore difficult the process from the industrial point of view and making it expensive from the economic point of view.
Besides values of extractable Fxe2x88x92 lower than or equal to 1 ppm, good mechanical properties both at low (23xc2x0 C.) and high temperature (250xc2x0 C.), good flex life, it would be desirable to carry out the processing of the manufactured articles, for example pipes, without compromising the productivity thereof. At present in the preparation of pipes having a low release of Fxe2x88x92 for the semicon industry, perfluoropolymers post-treated as above indicated are absolutely necessary and not high extrusion rates are used.
The need was therefore felt to have available in the semicon industry a thermoprocessable fluoropolymer able to give finished manufactured articles having the following combination of proprties:
release values of Fxe2x88x92 ions, both on the polymer pellet and on the finished manufactured article, lower than 1 ppm by weight with respect to the polymer weight;
said release values of Fxe2x88x92 ions lower than 1 ppm, in the case of extruded manufactured articles (pipes), are obtainable also at high extrusion rate;
very low values of extractable Clxe2x88x92, lower than 0.2 ppm by weight with respect to the polymer weight, preferably lower than the analytical limits;
flex life values higher than 20,000 combined with a MFI range (measured at 372xc2x0 C. with a 5 Kg load) comprised between 1 and 5.
very good mechanical properties both at low temperature (23xc2x0 C.) and at high temperature (250xc2x0 C.).
The Applicant has surprisingly and unexpectedly found that it is possible to obtain the combination of the above properties by using the specific monomeric composition of thermoprocessable copolymers of TFE as defined hereunder.
An object of the present invention is therefore a thermoprocessable copolymer of TFE consisting essentially of:
(a) from 4.5 to 8.5% by weight of perfluoromethylvinylether (PMVE);
(b) from 0.3 to 1.6% by weight of a perfluorodioxole of formula: 
xe2x80x83wherein:
Yxe2x95x90F, ORf, Rf wherein Rf is a perfluoroalkyl having from 1 to 5 carbon atoms;
X1 and X2, equal to or diferent from each other, are xe2x80x94F or xe2x80x94CF3;
(c) TFE, forming the remaining part to 100%; said copolymer having:
a number of ionic end groups lower than 4xc3x9710xe2x88x924 mol/Kg of polymer;
flex life values higher than 20,000 in a MFI range (measured at 372xc2x0 C. with a 5 Kg load) comprised between 1 and 5;
release values of Fxe2x88x92 ions, both on the polymer in pellet and on the finished manufactured article, lower than 1 ppm by weight with respect to the polymer weight;
release values of Fxe2x88x92 ions lower than 1 ppm on extruded manufactured articles (pipes), obtained both at low and high extrusion rate of the polymer.
The ionic end groups are for example of xe2x80x94COOH and xe2x80x94COF type.
Among the comonomers (b) the one wherein Yxe2x95x90F, X1, X2xe2x95x90CF3 can for example be mentioned; said compound is perfluoro-2,2-dimethyl-1,3-dioxole (PDD). See for example U.S. Pat. No. 3,865,845.
Preferably in the present invention the compound of formula (I) wherein Yxe2x95x90ORf where Rfxe2x95x90xe2x80x94CF3; X1, X2xe2x95x90F is used as monomer (b). Said compound is named 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD).
The copolymers of the invention, besides showing very good mechanical properties both at hot and cold as above mentioned, and very good flex life, unexpectedly do not require any treatment of fluorination or transformation of the end groups of the type of those above described, for obtaining extractable values of Fxe2x88x92 lower than 1 ppm on the finished manufactured articles. This result in the case of extruded manufactured articles, for example pipes, is surprising since it is obtainable at various extrusion rates of the manufactured article and do not penalize the productivity during the material processing (see the Examples). In fact one can operate also at an extrusion rate double than those at present used in the manufactured article processing for the semicon industry, without having a release of Fxe2x88x92 ions higher than 1 ppm. This is extremely important from the industrial point of view since the productivity is doubled.
To obtain the above combination of properties which makes the finished manufactured articles suitable to be used in the semicon industry, it has been found by the Applicant that it is necessary to use in polymerization comonomers (a) and (b) in the above defined amounts so that the amount of ionic end groups is the above mentioned one. In fact, tests carried out by the Applicant (see the comparative Examples) show that when the concentrations of perfluorodioxole or perfluoromethylvinylether are higher than the indicated limits, there is a worsening of the mechanical properties in hot conditions, in particular the stress at break, measured at 250xc2x0 C., becomes lower than 5 MPa and the elongation at break (at 250xc2x0 C.) becomes lower than 350%, which are the minimum acceptable values for a commercial fluoropolymer HYFLON(copyright) MFA 620. When on the contrary the concentrations of perfluoromethylvinylether or perfluorodioxole are lower than the indicated values, the finished polymer shows low flex life values and therefore it does not result satisfactory in the applications of semicon industry.
When the copolymers of the invention are used for obtaining pipes by extrusion, they preferably have a MFI in the range 1-5 measured at 372xc2x0 C. with a 5 kg load.
When the copolymers of the invention are used for obtaining by moulding fittings, connections for pipings, etc., they preferably have a MFI comprised between 6 and 30 measured at 372xc2x0 C. with a 5 kg load.
The copolymers of the invention can be obtained by polymerizing the monomers by radical route both in aqueous and in organic medium. The polymerization in aqueous medium can be carried out in emulsion or in microemulsion in the presence of a radical inorganic initiator such as for example the ammonium and/or potassium and/or sodium persulphate, optionally in combination with ferrous, cuprous or silver salts. The initiator feeding can be made in a continuous way or by a single addition at the starting of the polymerization. This latter method is preferable since it reduces the polymerization times, the ionic end groups in the polymer being equal. It has been found by the Applicant that the used initiator amount must be low. For example by operating at a temperature of 75xc2x0 C. with a pressure of 22 absolute bar, the initiator amount, all fed at the starting of the polymerization, and in an amount lower than 0.03 grams of potassium persulphate for liter of water, allows to obtain the above indicated number of ionic end groups.
To obtain the results of the present invention it is preferable that the perfluorodioxole monomer (b) is fed in a continuous way during the whole polymerization.
The synthesis temperature can be in the range 25xc2x0-120xc2x0 C. A temperature range 60xc2x0-95xc2x0 C. is preferred when polymerization is carried out in aqueous emulsion or microemulsion in the presence of persulphates. The polymerization can take place at pressures in the range 10-50 bar.
The polymerization in aqueous medium requires the presence of a surfactant, fluorinated surfactants such as perfluorooctanoate or ammonium, potassium or sodium perfluorooctanoate, perfluorononanoate, perfluorodecanoate mixtures are particularly preferred. It is particularly suitable to carry out the polymerization in aqueous phase in the presence of perfluoropolyethers as surfactants. Such perfluoropolyethers can be added to the reaction medium under the form of a microemulsion, as described in U.S. Pat. No. 4,864,006.
For the control of the molecular weight of the invention terpolymers, chain transfer agents such as hydrogen, methane, ethane, propane are used; which give end groups of hydrogenated type, suitable therefore to the applications of the invention in the semicon industry. Chlorinated transfer agents are not used since it has been found that they produce amounts of extractable Clxe2x88x92 dangerous for the semicon industry.
The polymerization latex is coagulated with a coagulant such as for example nitric acid; then the slurry washing and the subsequent drying of the wet polymer take place. The powder is then pelletized in a twin-screw extruder equipped with at least two degassing zones.
The present invention will be better illustrated by the following Examples, which have a merely indicative but not limitative purpose of the scope of the invention itself.